GM Code 51 PROM Fault

When the PROM was initially created, a checksum number representing the total of all the cell values was also stored. The ECM uses this checksum as one way to determine if the contents of any cell has changed. A Code 51 causes the ECM to enter Back-Up mode which uses the CALPAK for default sensor values.

The conditions for setting this code are:

power has just been applied to the ECM, and

the calculated checksum does not match the stored checksum.

- or -

the engine was just started, and

either of two specific PROM locations do not contain the expected value. (One location should have 1's in the odd numbered bits and the other has 1's in the even numbered bit positions.)